This invention is directed toward the measure of geophysical parameters of earth formations penetrated by a borehole. The invention employs propagation resistivity techniques utilizing a downhole instrument comprising multiple, longitudinally spaced transmitters operating at different frequencies with a plurality of longitudinally spaced receiver pairs. An electromagnetic wave is propagated from the transmitting antenna coil into the formation in the vicinity of the borehole and detected as it passes the receiving antenna of the receiver pair. The basic parameters measured at the receivers are the amplitude and phase shift of the sensed electromagnetic wave. The downhole instrument is conveyed along the borehole by a drill string or other means thereby making the basic measurements as a function of position or depth of the downhole instrument within the borehole. A plurality of parameters of interest can be determined by combining the basic measurements. Such parameters include the resistivity, dielectric constant and porosity of the formation as well as the degree to which the fluid within the borehole migrates into or "invades" the virgin formation. Numerous factors affect the accuracy and precision of the desired parametric measurements. These include, but are not limited to, the radial position of the measuring device within the well bore, the shape or eccentricity of the borehole, the type of borehole fluid and the electrical and mechanical characteristics of the transmitters and receivers. In addition the amplitude and phase measurements made at different transmitter-receiver spacings or at different transmitter frequencies exhibit different responses to vertical changes in the formation as the instrument is conveyed along the borehole. The invention is further directed toward the combination of multiple electromagnetic propagation measurements to obtain more accurate and precise measurements of formation resistivity, dielectric constant, porosity and borehole fluid invasion profile when the perturbing effects of the borehole environs and vertical resolution properties of the transmitter receiver combinations have been minimized. The invention is still further directed toward the measure of the properties of the well bore itself which may be used to evaluate mechanical properties of the rock and the effectiveness of the drilling program.
Induction techniques have been used for a number of years to determine the resistivity of earth formations penetrated by a borehole. Historically, formation resistivity has been used to delineate hydrocarbons from saline formation waters. Resistivity cannot, however, be used to delineate hydrocarbon from relatively fresh formation waters since both fluids exhibit very high resistivity. The resistivity contrast between the resistivity of hydrocarbon and fresh water is less than the precision of borehole resistivity measurement systems. Hydrocarbons and waters, both saline and fresh, do exhibit measurable contrast in dielectric constant. Dielectric constant can therefore be used to delineate hydrocarbons from fresh waters or waters of unknown salinity as well as will be discussed later. Using induction techniques to measure formation parameters, an alternating current is applied to one or more transmitters of the borehole instrument thereby generating an electromagnetic field in the vicinity of the transmitter. The primary field interacts with the earth formation thereby setting up secondary fields with the amplitude and the phase of the secondary fields being related to electromagnetic properties of the formation. Amplitude and phase are the primary or "raw" parameters measured by the receivers. These raw measurements are combined to obtain the parameters of interest and to eliminate unwanted noise as will be detailed in this disclosure.
As mentioned previously amplitude and phase measurements made at different transmitter receiver spacings and at different frequencies exhibit different vertical resolutions. Prior art has matched the vertical resolutions using various convolution and deconvolution techniques prior to combining multiple measurements. This is referred in the art as "serial" data processing. U.S. Pat. No. 4,609,873 to Percy T. Cox, et al teaches the use of a wireline logging system comprising at least three transmitter coils and at least two receiver coils to determine resistivity and dielectric constant of a subsurface formation adjacent to a drilling fluid invaded zone. The transmitters are operated at a single frequency of 30 MHz. Amplitude and phase measurements are made and serial processing of the data is employed. At relatively low transmitter frequencies, serial processing introduces only negligible errors. At higher transmitter frequencies in the 2 MHz range or higher, vertical resolution is affected not only by the physical arrangement of the transmitter receiver combinations, but also significantly by the electromagnetic properties of the borehole environments and the formation. The functional dependence of vertical resolution and transmitter frequency is addressed in the publication "2-MHz Propagation Resistivity Modeling in Invaded Thin Beds", W. Hal Meyer, The Log Analyst, July-August 1993, p. 33 and "Inversion of 2 MHz Propagation Resistivity Logs", W. H. Meyer, SPWLA 33rd Annual Logging Symposium, Paper H, Jun. 14-17, 1992. Stated another way, serial processing of data can introduce significant error at transmitter frequencies in the range of 2 MHz and higher. In order to obtain accurate and precise parametric determinations at these frequencies, it is necessary to compute the parameters of interest and to make the required corrections, including corrections for the effects of differing vertical resolutions, simultaneously. Methods for accomplishing this goal will be detailed in this disclosure.